In U.S. Pat. No. 4,113,360 to Bauer, a display device is described comprising a first plate acting as a light guide or fluorescent material, a second plate positioned some distance apart from the first plate, and a thin movable microfilm situated between the two plates. As used herein microfilm means a thin flexible film less than 500 microns thick. The movable microfilm is flexible and can be made to locally contact the first plate and allow light to be transmitted from the first plate into the microfilm. If the microfilm is constructed to scatter the light, then movable microfilm acts as an optical switch to create bright and dark regions on the plates as the microfilm contacts or separates from the first plate, respectively. Rapid contact and separation between the microfilm and the first plate can be used to create gray regions.
As described in U.S. Pat. No. 4,113,360 to Bauer, the motion of the microfilm can be controlled by electrical means. For example, the microfilm may contain a very thin layer of indium tin oxide that permits an electrical charge to be applied to the microfilm. Similar conductive layers may be placed on the plates. An electrical bias between the plates and the microfilm may be used to move the microfilm toward or away from the light guide. Alternatively, U.S. Pat. No. 5,771,321 to Stern, describes an electromechanical means of controlling the movement of the microfilm.
Typically, the plates are rigid with a thickness on the order of millimeters and are comprised of clear materials such as glass or plastic (e.g. Plexiglas or polycarbonate). The microfilm, on the other hand, must be flexible and has thickness on the order of a micron. The microfilm may be comprised of resin material such as polycarbonate or polystyrene as suggested in U.S. Pat. No. 5,771,321 to Stern.
One drawback to preparing an information display panel using the optical switching device described above, is the need for an economical and simple method to manufacture the flexible microfilm. U.S. Pat. No. 5,771,321 to Stern describes a means of creating a rough surfaced microfilm by dipping a sheet of the microfilm into a solution of spheres. When the sheet is removed from the solution, the spheres are adhered to the sheet by surface tension. The microfilm is then heated to permanently fix the spheres to the sheet. The resulting irregular surface is said to be a light scattering surface. However, U.S. Pat. No. 5,771,321 to Stern does not describe how to prepare the thin precursor sheets. Moreover, U.S. Pat. No. 5,771,321 to Stern does not provide a method of controlling the roughness of each side of the microfilm independently. In addition, it may desirable to prepare a microfilm with an internal light scattering means as well as a surface scattering means. It is also desirable to have microfilms prepared with low birefringence. The preparation of such a microfilm for optical switch applications has not been described.
Resin microfilms used to prepare the various types of optical components described above are generally desired to have good light scattering abiltiy, transparency, high uniformity, and low birefringence. Moreover, these microfilms may be needed in a range of thickness depending on the final application.
In general, resin microfilms are prepared either by melt extrusion methods or by casting methods. Melt extrusion methods involve heating the resin until molten (approximate viscosity on the order of 100,000 cp), and then applying the hot molten polymer to a highly polished metal band or drum with an extrusion die, cooling the film, and finally peeling the film from the metal support. For many reasons, however, films prepared by melt extrusion are generally not suitable for optical applications. Principal among these is the fact that melt extruded films exhibit a high degree of optical birefringence. In the case of highly substituted cellulose acetate, there is the additional problem of melting the polymer. Cellulose triacetate has a very high melting temperature of 270–300° C., and this is above the temperature where decomposition begins. Films have been formed by melt extrusion at lower temperatures by compounding cellulose acetate with various plasticizers as taught in U.S. Pat. No. 5,219,510 to Machell. However, the polymers described in U.S. Pat. No. 5,219,510 to Machell are not the fully substituted cellulose triacetate, but rather have a lesser degree of alkyl substitution or have proprionate groups in place of acetate groups. Even so, melt extruded films of cellulose acetate are known to exhibit poor flatness as noted in U.S. Pat. No. 5,753,140 to Shigenmura. For these reasons, melt extrusion methods are generally not practical for fabricating many resin films including cellulose triacetate films. Rather, casting methods are generally used to manufacture these films. In general, resin films are prepared either by melt extrusion methods or by casting methods. Melt extrusion methods involve heating the resin until molten (approximate viscosity on the order of 100,000 cp), and then applying the hot molten polymer to a highly polished metal band or drum with an extrusion die, cooling the film, and finally peeling the film from the metal support. For many reasons, however, films prepared by melt extrusion are generally not suitable for optical applications. Principal among these is the fact that melt extruded films exhibit a high degree of optical birefringence. In the case of highly substituted cellulose acetate, there is the additional problem of melting the polymer. Cellulose triacetate has a very high melting temperature of 270–300° C., and this is above the temperature where decomposition begins. Films have been formed by melt extrusion at lower temperatures by compounding cellulose acetate with various plasticizers as taught in U.S. Pat. No. 5,219,510 to Machell. However, the polymers described in U.S. Pat. No. 5,219,510 to Machell are not the fully substituted cellulose triacetate, but rather have a lesser degree of alkyl substitution or have proprionate groups in place of acetate groups. Even so, melt extruded films of cellulose acetate are known to exhibit poor flatness as noted in U.S. Pat. No. 5,753,140 to Shigenmura. For these reasons, melt extrusion methods are generally not practical for fabricating many resin films including cellulose triacetate films used to prepare protective covers and substrates in electronic displays. Rather, casting methods are generally used to manufacture these films.
A prior art method of casting resin microfilms is illustrated in FIG. 8. As shown in FIG. 8, a viscous polymeric dope is delivered through a feed line 200 to an extrusion hopper 202 from a pressurized tank 204 by a pump 206. The dope is cast onto a highly polished metal drum 208 located within a first drying section 210 of the drying oven 212. The cast microfilm 214 is allowed to partially dry on the moving drum 208 and is then peeled from the drum 208. The cast microfilm 214 is then conveyed to a final drying section 216 to remove the remaining solvent. The final dried microfilm 218 is then wound into rolls at a wind-up station 220. The prior art cast microfilm typically has a thickness in the range of from 40 to 200 μm.
In general, thin microfilms of less than 40 μm are very difficult to produce by casting methods due to the fragility of wet microfilm during the peeling and drying processes. Cast microfilms may exhibit undesirable cockle or wrinkles. Thinner microfilms are especially vulnerable to dimensional artifacts either during the peeling and drying steps of the casting process or during subsequent handling of the microfilm. In addition, many cast microfilms may naturally become distorted over time due to the effects of moisture. For optical microfilms, good dimensional stability is necessary during storage as well as during subsequent assembly. Melt extruded microfilms have many of the same problems as cast microfilms. In addition only certain polymeric materials may be used to produce melt-extruded microfilms because the heat used to liquify the polymer may degrade the polymer.
There is a need, therefore, for an improved method of making resin microfilms for use as optical switch components.